This invention relates to the packaging of food products, and more particularly to a method and apparatus for forming hermetically sealed packages which have been purged of air by multiple flushing of the packages with inert gas to retain the food products in a fresh state.
It is known in the art of packaging processed food to evacuate air from the packages by the application of subatmospheric pressure commonly called drawing a vacuum, and thereafter hermetically sealing the packages to retain the original freshness of the product. Since a perfect vacuum is substantially impossible to attain within the earth's atmosphere, and since low subatmospheric pressures are difficult and costly to attain, it is common, when packaging certain food products, to flush the packages with relatively inert gas which substantially displaces the air. This flushing may be used together with or in lieu of the evacuation step. However, unless a substantial amount of gas is utilized, a certain amount of air, which contains various food spoiling impurities, remains in the packages resulting in a shortened shelf life. In order to increase the amount of purging gas supplied to the packages and thereby flush more air from the packages utilizing prior art methods and apparatus would require that the processing time be substantially increased, thereby increasing the packaging cost. The disadvantage of increased cost appears not to be justified by the increased shelf life of the product's package. Consequently, a single application of gas to flush the packages is the manner in which the prior art has developed.
In the formation of the packages, a thermoplastic material is heated and formed by dyes at a forming station to form a base having one or more food containing cavities. The base is moved to a food dispensing station and the food is placed into the cavities. It then moves to a station wherein a covering of lidding material is laid over the base. The evacuation and/or flushing thereafter is performed and the lidding material and base are pressed together and sealed. There are generally two methods for applying the flushing gas to the packages. One of the method applies the gas through a series of hollow pins disposed in a slit made in the forming material or the lidding material, the pins being intermediate the sealing heater and thus the later sealed portions of the materials and small clips outside of the seal area which hold the materials together while the gas is applied. Since the pins must of necessity be of a relatively small diameter, a small area is presented through which the gas flows and thus the gasing time is relatively long. Additionally, the slit has to be sealed. An improvement to this method is the use of a nozzle system whereby the gas is applied through one or more nozzles and directed between the forming material and the lidding material outside of the subsequently sealed area. Although this method provides a faster flush because of the larger area through which the gas may flow, the available area is still relatively small so that the gasing time is still relatively large.